I have several free style projects and i am seeing this error from last couple of days Randomly . Please find below the error trace . I did not come across any working solution anywhere . Please help !!

Started by an SCM change
Building on master in workspace /home/cmsbuild/Workspace/middletier
> /usr/bin/git rev-parse --is-inside-work-tree # timeout=10
Fetching changes from the remote Git repository
> /usr/bin/git config remote.origin.url git@gitlab.ins.risk.regn.net:cmd/middletier.git # timeout=10
Fetching upstream changes from git@gitlab.ins.risk.regn.net:cmd/middletier.git
> /usr/bin/git --version # timeout=10
using GIT_SSH to set credentials
> /usr/bin/git -c core.askpass=true fetch --tags --progress git@gitlab.ins.risk.regn.net:cmd/middletier.git +refs/heads/:refs/remotes/origin/
> /usr/bin/git rev-parse refs/remotes/origin/test_CI^

# timeout=10
Checking out Revision a2fa4a0cfae39d50d51d01d46190891f879b6d8a (refs/remotes/origin/test_CI)
> /usr/bin/git config core.sparsecheckout # timeout=10
> /usr/bin/git checkout -f a2fa4a0cfae39d50d51d01d46190891f879b6d8a
> /usr/bin/git rev-list 2b4d0c33e20ba201fd0896b75223555f005dde98 # timeout=10
FATAL: hudson/tasks/Mailer$UserProperty
java.lang.NoClassDefFoundError: hudson/tasks/Mailer$UserProperty
at hudson.plugins.git.GitChangeLogParser.parseCommit(GitChangeLogParser.java:77)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:71)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:48)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:29)
at hudson.model.AbstractBuild.calcChangeSet(AbstractBuild.java:927)
at hudson.model.AbstractBuild.access$600(AbstractBuild.java:104)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:619)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:529)
at hudson.model.Run.execute(Run.java:1738)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)
Caused by: java.lang.ClassNotFoundException: hudson.tasks.Mailer$UserProperty
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1376)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1326)
at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1079)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 13 more
ERROR: Step ‘E-mail Notification’ aborted due to exception:
java.lang.NoClassDefFoundError: hudson/tasks/Mailer$UserProperty
at hudson.plugins.git.GitChangeLogParser.parseCommit(GitChangeLogParser.java:77)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:71)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:48)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:29)
at hudson.model.AbstractBuild.calcChangeSet(AbstractBuild.java:927)
at hudson.model.AbstractBuild.getChangeSet(AbstractBuild.java:895)
at hudson.tasks.MailSender.getChangeSet(MailSender.java:215)
at hudson.tasks.MailSender.appendBuildUrl(MailSender.java:252)
at hudson.tasks.MailSender.createFailureMail(MailSender.java:267)
at hudson.tasks.MailSender.createMail(MailSender.java:180)
at hudson.tasks.MailSender.run(MailSender.java:109)
at hudson.tasks.Mailer.perform(Mailer.java:170)
at hudson.tasks.BuildStepCompatibilityLayer.perform(BuildStepCompatibilityLayer.java:76)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:782)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:723)
at hudson.model.Build$BuildExecution.post2(Build.java:185)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:668)
at hudson.model.Run.execute(Run.java:1763)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)
Caused by: java.lang.ClassNotFoundException: hudson.tasks.Mailer$UserProperty
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1376)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1326)
at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1079)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 22 more
FATAL: hudson/tasks/Mailer$UserProperty
java.lang.NoClassDefFoundError: hudson/tasks/Mailer$UserProperty
at hudson.plugins.git.GitChangeLogParser.parseCommit(GitChangeLogParser.java:77)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:71)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:48)
at hudson.plugins.git.GitChangeLogParser.parse(GitChangeLogParser.java:29)
at hudson.model.AbstractBuild.calcChangeSet(AbstractBuild.java:927)
at hudson.model.AbstractBuild.getChangeSet(AbstractBuild.java:895)
at hudson.model.AbstractBuild.getCulprits(AbstractBuild.java:349)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:672)
at hudson.model.Run.execute(Run.java:1763)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:410)
Caused by: java.lang.ClassNotFoundException: hudson.tasks.Mailer$UserProperty
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1376)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1326)
at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1079)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 12 more
Finished: FAILURE